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- Author or Editor: W. L. Ecklund x
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Abstract
Radars that can make wind measurements in the clear air are expected to play an increasing role in meteorological observing systems in the future, especially for horizontal wind measurements. This paper considers the prospects for using these radars, which are sometimes called wind profilers, to also measure the large‐scale vertical velocity. Unfortunately, all radars for which vertical velocity data are available at this time are located in or near mountains, where standing lee‐wave effects often make the data representative of only small‐scale features. Confining attention to those times when lee wave effects are not expected, case‐study comparisons of the existing radar data with indirectly computed synoptic‐scale motions suggest that time averaged radar data are representative of large‐scale features smaller than the synoptic scale, perhaps more aptly termed subsynoptic‐scale features. Results from a three‐station radar network in France show that the time‐averaged vertical velocities are usually nearly the same at all stations, although there are some differences, and suggest that the spatial scale of the flow features they represent is greater than 50 km. Over a long-term average, the net influence of lee wave effects at mountain sites is small, and radar measurements appear to be useful for climatological studies of vertical velocity in large‐scale circulation systems.
Abstract
Radars that can make wind measurements in the clear air are expected to play an increasing role in meteorological observing systems in the future, especially for horizontal wind measurements. This paper considers the prospects for using these radars, which are sometimes called wind profilers, to also measure the large‐scale vertical velocity. Unfortunately, all radars for which vertical velocity data are available at this time are located in or near mountains, where standing lee‐wave effects often make the data representative of only small‐scale features. Confining attention to those times when lee wave effects are not expected, case‐study comparisons of the existing radar data with indirectly computed synoptic‐scale motions suggest that time averaged radar data are representative of large‐scale features smaller than the synoptic scale, perhaps more aptly termed subsynoptic‐scale features. Results from a three‐station radar network in France show that the time‐averaged vertical velocities are usually nearly the same at all stations, although there are some differences, and suggest that the spatial scale of the flow features they represent is greater than 50 km. Over a long-term average, the net influence of lee wave effects at mountain sites is small, and radar measurements appear to be useful for climatological studies of vertical velocity in large‐scale circulation systems.
Abstract
Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.
Abstract
Recently published data (Ecklund and Balsley) describing VHF radar echo characteristics from the Arctic mesosphere and lower thermosphere show a remarkable seasonal dependence of both the echo height and echo intensity: during the three-month period around the summer solstice, intense and nearly continuous echoes are returned from a narrow (±2 km half-power) region centered at 86 km; during the remainder of the year, however, the echoes are much weaker, more sporadic and occur at a much lower altitude (70 km ± 9 km). In this paper, we present additional data that suggest that the summer echoes are primarily the result of shear instability of low-frequency (tidal) motions in the region of high stratification above the Arctic summer mesopause, while the winter echoes arise from the nonlinear breakup of upward-propagating gravity waves.
Abstract
In this paper we describe a boundary layer radar recently developed at NOAA's Aeronomy Laboratory. This radar extends wind profiler technology by using a small, relatively inexpensive radar to provide continuous, high-resolution wind measurements in the first few kilometers of the atmosphere. Although the radar was developed for use in a “hybrid” mode with existing 50 MHz profilers in the tropical Pacific, the system can equally well be a stand-alone device to study boundary layer problems.
Abstract
In this paper we describe a boundary layer radar recently developed at NOAA's Aeronomy Laboratory. This radar extends wind profiler technology by using a small, relatively inexpensive radar to provide continuous, high-resolution wind measurements in the first few kilometers of the atmosphere. Although the radar was developed for use in a “hybrid” mode with existing 50 MHz profilers in the tropical Pacific, the system can equally well be a stand-alone device to study boundary layer problems.
Abstract
During March 1981 the Sunset and Platteville VHF clear-air radars located in Colorado to the east of the continental divide observed vertical winds continuously over a three-week period. The vertical winds at these locations contain fluctuations with periods from a few minutes to several hours and with magnitudes ranging up to a few meters per second. The Sunset radar, which is located in the foothills, observed systematically larger vertical velocities than the vertical velocities observed by the Platteville radar, which is located on the plains, some 60 km to the east. Although periods of enhanced vertical wind activity were observed to occur at the same time at both sites, attempts to correlate vertical wind structures over the two sites in detail were generally not successful.
The magnitude of vertical velocity fluctuations seen by both radars show large day-to-day variations with “active” periods alternating with “quiet” periods. An examination of upper level maps reveals that the occurrence of active and quiet periods are linked to the large-scale wind field. During the March experiment the magnitude of the vertical velocity variance was well correlated with the 500 mb zonal (west) wind.
Abstract
During March 1981 the Sunset and Platteville VHF clear-air radars located in Colorado to the east of the continental divide observed vertical winds continuously over a three-week period. The vertical winds at these locations contain fluctuations with periods from a few minutes to several hours and with magnitudes ranging up to a few meters per second. The Sunset radar, which is located in the foothills, observed systematically larger vertical velocities than the vertical velocities observed by the Platteville radar, which is located on the plains, some 60 km to the east. Although periods of enhanced vertical wind activity were observed to occur at the same time at both sites, attempts to correlate vertical wind structures over the two sites in detail were generally not successful.
The magnitude of vertical velocity fluctuations seen by both radars show large day-to-day variations with “active” periods alternating with “quiet” periods. An examination of upper level maps reveals that the occurrence of active and quiet periods are linked to the large-scale wind field. During the March experiment the magnitude of the vertical velocity variance was well correlated with the 500 mb zonal (west) wind.
Abstract
Wind profilers are radars that operate in the VHF and UHF hands and are designed for detecting the weak echoes reflected by the optically clear atmosphere. An unexpected application of wind profilers has been the revival of an old method of estimating drop size distributions in rain from the Doppler spectrum of the received signal. Originally attempted with radars operating at microwave frequency, the method showed early promise but was seriously limited in application because of the crucial sensitivity of the estimated drop sizes to the vertical air velocity, a quantity generally unknown and, at that time, unmeasurable. Profilers have solved this problem through their ability to measure, under appropriate conditions, both air motions and drop motions. This paper compares the drop sizes measured by a UHF profiler at two altitudes in a shower with those measured simultaneously by an instrumented airplane. The agreement is satisfactory, lending support to this new application of wind profilers.
Abstract
Wind profilers are radars that operate in the VHF and UHF hands and are designed for detecting the weak echoes reflected by the optically clear atmosphere. An unexpected application of wind profilers has been the revival of an old method of estimating drop size distributions in rain from the Doppler spectrum of the received signal. Originally attempted with radars operating at microwave frequency, the method showed early promise but was seriously limited in application because of the crucial sensitivity of the estimated drop sizes to the vertical air velocity, a quantity generally unknown and, at that time, unmeasurable. Profilers have solved this problem through their ability to measure, under appropriate conditions, both air motions and drop motions. This paper compares the drop sizes measured by a UHF profiler at two altitudes in a shower with those measured simultaneously by an instrumented airplane. The agreement is satisfactory, lending support to this new application of wind profilers.
Abstract
A 915-MHz boundary-layer wind profiler radar with radio acoustic sounding system (RASS) capability has been used to measure the turbulent fluxes of heat and momentum in the convective boundary layer by eddy correlation. The diurnal variation of the heat flux at several heights between 160 and 500 m above ground level and values of the momentum flux for 2-h periods in midday from 160 to 1000 m are presented, as well as wind and temperature data. The momentum flux is calculated both from the clear-air velocities and from the RASS velocities, and the two results are compared.
Abstract
A 915-MHz boundary-layer wind profiler radar with radio acoustic sounding system (RASS) capability has been used to measure the turbulent fluxes of heat and momentum in the convective boundary layer by eddy correlation. The diurnal variation of the heat flux at several heights between 160 and 500 m above ground level and values of the momentum flux for 2-h periods in midday from 160 to 1000 m are presented, as well as wind and temperature data. The momentum flux is calculated both from the clear-air velocities and from the RASS velocities, and the two results are compared.
Abstract
Multiheight time series of atmospheric vertical velocities in the troposphere and lower stratosphere observed by clear-air Doppler radar are presented at various locations around the world. Frequency spectra of vertical velocities determined from these data sets are compared with the objective of developing a preliminary climatology. We emphasize the nearly universal shape and magnitude of spectra observed during low-wind conditions. These spectra are quite flat for frequencies between the buoyancy and inertial frequencies, and they closely resemble the internal wave spectra observed in the ocean. Spectra observed under strong wind conditions, on the other hand, are greatly enhanced in magnitude, approaching the f −5/3 spectral slope observed for the spectrum of horizontal motions. Finally, spectra determined from both quiet and active periods at Poker Flat, Alaska, possess spectral slopes and amplitudes intermediate to those spectra determined solely from quiet or active periods at other locations.
Abstract
Multiheight time series of atmospheric vertical velocities in the troposphere and lower stratosphere observed by clear-air Doppler radar are presented at various locations around the world. Frequency spectra of vertical velocities determined from these data sets are compared with the objective of developing a preliminary climatology. We emphasize the nearly universal shape and magnitude of spectra observed during low-wind conditions. These spectra are quite flat for frequencies between the buoyancy and inertial frequencies, and they closely resemble the internal wave spectra observed in the ocean. Spectra observed under strong wind conditions, on the other hand, are greatly enhanced in magnitude, approaching the f −5/3 spectral slope observed for the spectrum of horizontal motions. Finally, spectra determined from both quiet and active periods at Poker Flat, Alaska, possess spectral slopes and amplitudes intermediate to those spectra determined solely from quiet or active periods at other locations.
Abstract
Average vertical profiles of the vertical wind obtained under clear sky conditions as weal as under conditions of both light-to-moderate and heavy rainfall am presented from data obtained using a radar wind profiler located on the island of Pohnpei (latitude 7°N, longitude 157°E). The average profiles for the precipitation conditions were obtained, insofar as possible, under conditions similar to those present within the stratiform and convective regions of tropical mesoscale convective complexes. Comparison between the vertical wind profiles obtained from the wind profiler and vertical wind profiles obtained earlier by wore conventional methods (i.e., deduced from the convergence-divergence of mesoscale horizontal winds) shows that, while the general features of the profiles obtained by both techniques are similar, the profiler results exhibit somewhat more detail. The profiler is able to resolve long-term average vertical motions down to the, ∼cm s−1 subsidence that occurs under clear air conditions. Additional evidence for an apparent difference between vertical wind profiles in the Atlantic and Pacific regions in heavy convection reported earlier, is presented.
Abstract
Average vertical profiles of the vertical wind obtained under clear sky conditions as weal as under conditions of both light-to-moderate and heavy rainfall am presented from data obtained using a radar wind profiler located on the island of Pohnpei (latitude 7°N, longitude 157°E). The average profiles for the precipitation conditions were obtained, insofar as possible, under conditions similar to those present within the stratiform and convective regions of tropical mesoscale convective complexes. Comparison between the vertical wind profiles obtained from the wind profiler and vertical wind profiles obtained earlier by wore conventional methods (i.e., deduced from the convergence-divergence of mesoscale horizontal winds) shows that, while the general features of the profiles obtained by both techniques are similar, the profiler results exhibit somewhat more detail. The profiler is able to resolve long-term average vertical motions down to the, ∼cm s−1 subsidence that occurs under clear air conditions. Additional evidence for an apparent difference between vertical wind profiles in the Atlantic and Pacific regions in heavy convection reported earlier, is presented.
Abstract
Improved radio acoustic sounding system (RASS) technology for use with radar wind profilers has been developed and applied to 915-MHz and 50-MHz profilers. The most important advance is the simultaneous measurement of the wind velocity to correct the acoustic velocity measurement for air motion. This eliminates the primary source of error in previous RASS measurements, especially on short time scales. Another improvement is the use of an acoustic source that is controlled by the same computer that controls the radar. The source can be programmed to produce either a swept frequency or a random hopped frequency signal. Optimum choices of the acoustic source parameters are explored for particular applications. Simultaneous measurement of acoustic and wind velocity enables the calculation of heat flux by eddy correlation. Preliminary heat flux measurements are presented and discussed. Results of the use of RASS with oblique beams are also reported.
Abstract
Improved radio acoustic sounding system (RASS) technology for use with radar wind profilers has been developed and applied to 915-MHz and 50-MHz profilers. The most important advance is the simultaneous measurement of the wind velocity to correct the acoustic velocity measurement for air motion. This eliminates the primary source of error in previous RASS measurements, especially on short time scales. Another improvement is the use of an acoustic source that is controlled by the same computer that controls the radar. The source can be programmed to produce either a swept frequency or a random hopped frequency signal. Optimum choices of the acoustic source parameters are explored for particular applications. Simultaneous measurement of acoustic and wind velocity enables the calculation of heat flux by eddy correlation. Preliminary heat flux measurements are presented and discussed. Results of the use of RASS with oblique beams are also reported.
Abstract
The magnitude of backscattered power observed at vertical incidence by a VHF radar is related to atmospheric stability in accordance with the Fresnel scattering model. Utilizing a modified Fresnel scattering model, we can determine tropopause height objectively from the observed vertical profile of backscattered power. The method is tested with observations of the Alpine Experiment (ALPEX; France), Platteville, Colorado and Poker Mat, Alaska radars taken since 1979. Using 750 m resolution the tropopause is found to be within a few hundred meters of the tropopause determined from nearly simultaneous radiosonde observations and using 2.2 km resolution the tropopause is found to be within about 600 m. Furthermore, radar-determined tropopause heights can be automatically scaled from existing records, or even routinely determined on-line.
Abstract
The magnitude of backscattered power observed at vertical incidence by a VHF radar is related to atmospheric stability in accordance with the Fresnel scattering model. Utilizing a modified Fresnel scattering model, we can determine tropopause height objectively from the observed vertical profile of backscattered power. The method is tested with observations of the Alpine Experiment (ALPEX; France), Platteville, Colorado and Poker Mat, Alaska radars taken since 1979. Using 750 m resolution the tropopause is found to be within a few hundred meters of the tropopause determined from nearly simultaneous radiosonde observations and using 2.2 km resolution the tropopause is found to be within about 600 m. Furthermore, radar-determined tropopause heights can be automatically scaled from existing records, or even routinely determined on-line.
